In recent years, in the field of inverters for electric vehicles and the like, power semiconductor modules (such as IGBT or power MOSFET) are used that make a high-voltage/high-current operation possible. As a substrate used for power semiconductor modules, a ceramics circuit substrate can be used: The ceramic circuit substrate is made by joining a metallic circuit plate to one side of an insulating ceramics substrate and a metallic radiator plate to the other side. On the top surface of the metallic circuit plate, semiconductor elements and the like are mounted. So-called direct bonding copper method by which copper plates are directly joined, or active metal method that uses brazing filler metal, is adopted to join the above insulating ceramics substrate to the metallic circuit plate and the metallic radiator plate.
In such power semiconductor modules, the amount of heat generation increases as high current flows. However, the above insulating ceramics substrate is lower in thermal conductivity than the copper plate. Therefore, the above insulating ceramics substrate could play a role in preventing heat dissipation from the semiconductor elements. Moreover, based on a difference in the coefficient of thermal expansion between the insulating ceramics substrate and the metallic circuit and radiator plates, a thermal stress occurs, which could cause cracks on the insulating ceramics substrate and break the insulating ceramics substrate down, or cause the metallic circuit or radiator plate to come off from the insulating ceramics substrate. Therefore, to keep the excellent radiation performance of the insulating ceramics substrate, the high thermal conductivity and mechanical strength is required. A material of the insulating ceramics substrate is, for example, aluminum nitride or silicon nitride. An insulating ceramics substrate composed of aluminum nitride is high in thermal conductivity but low in mechanical strength, meaning such cracks could easily appear and that it is difficult for the insulating ceramics substrate to be used in a power semiconductor module having a structure in which a great amount of stress is applied to a ceramics substrate.
In PTL 1, which is mentioned below, an example of a silicon nitride substrate is disclosed. Twenty percent or more of a grain boundary phase is crystallized in order to reduce the proportion of a glassy phase that is low in thermal conductivity and increase the thermal conductivity of the silicon nitride substrate. Hereinafter, the above technique is referred to as a first conventional example. In PTL 2, which is mentioned below, an example of a silicon nitride ceramics material is disclosed. The grain boundary phase is made amorphous. Therefore, silicon nitride crystal grains are firmly combined thanks to the amorphous grain boundary phase, increasing the strength. Hereinafter, the above technique is referred to as a second conventional example. In PTL 3, which is mentioned below, an example of a silicon nitride radiator member is disclosed. Since the grain boundary phase contains a crystal phase made of MgSiO3 or MgSiN2, the high thermal conductivity silicon nitride radiator member is obtained. Hereinafter, the above technique is referred to as a third conventional example. In PTL 4, which is mentioned below, an example of a silicon nitride-based sintered body is disclosed. The grain boundary phase includes the crystal phase. It is reported that the sintered body is excellent in bending strength, fracture toughness and thermal shock resistance. Hereinafter, the above technique is referred to as a fourth conventional example.